Cold cathode fluorescent lamp (“CCFL”) backlighting for LCD displays, particularly those which require greater total luminous flux such as sunlight readable displays and LCD monitors, include secondary, non-imaging optical elements such as light pipes or integrating cavities. These secondary, non-imaging optical elements redistribute light from the CCFL into a more or less uniform area light source located behind the display.
LCD display backlighting systems using linear CCFL's advantageously exploit their characteristic that over most of its length a linear CCFL produces a uniformly illuminated cylinder of light. The uniform cylinder of light emitted by a CCFL can be readily transformed into an area source predominately by mounting the CCFL in an illuminator housing made of machine stamped metal such as aluminum or steel, and spreading the illumination in a linear fashion in a direction perpendicular to the CCFL tube's longitudinal axis. Since linear CCFLs are cylindrically radiating light sources, it proves difficult and inefficient to capture and direct all the light they emit exclusively onto the input aperture of an edge illuminated light pipe.
For secondary, non-imaging optical elements based upon a light pipe, also called edge-lit backlights, the CCFL's uniform cylinder of illumination conflicts, to some degree, with the flat strip nature of the light pipe's input surface, i.e., one or more edges of the light pipe. Efficiently transferring light emitted by a CCFL into a light pipe requires that the cross-sectional area of the light pipe's input surface should be roughly the same as the CCFL's cylindrical radiating area. However, the design of many modern electronic products that employ a backlit LCD require keeping the blacklight both as thin and as efficient as possible. This requirement for a concurrently thin and efficient LCD backlighting system could be readily solved if CCFLs could be made arbitrarily thin and still maintain its luminous output at a practical energizing voltage. Unfortunately, below about 2.0 mm diameter both the luminous output and practicality of CCFLs tend to decline precipitously. Consequently, most light pipe based backlighting systems tend to be inefficient, and to be dimmer in comparison with thicker, integrating cavity-type backlighting systems.
The highest efficiency and output mini CCFL based backlights used in currently available LCD modules typically omit the light pipe replacing it with an array of four (4) or more CCFL lamps located directly behind the LCD. Typically, the array of CCFLs is enclosed in a highly reflective integrating light box to generate and deliver increased luminous flux to the rear of the LCD. Unfortunately, these systems exhibit the following disadvantages.                1. Such CCFL illuminators add significant thickness to the overall LCD module, as much as 10 mm or more increased thickness.        2. These CCFL illuminators require using a high-output, multi-channel, high voltage inverter and lead wiring to drive the greater number of light sources. High-output, multi-channel, high voltage inverters tend to be heavy, bulky and often have moderately low electrical efficiency, 70%-80% efficiency is typical.        3. As is true for all CCFL light sources, at low ambient temperatures mini CCFL based backlights are increasingly difficult to start, and require:                    a. a very high start-up voltage, typically exceeding 1000 volts; and            b. a warm-up period that can be as long as several minutes before reaching their more efficient and higher output operating temperature range.                        4. CCFL lamps, including mini CCFLs, produced essentially all their heat inside their glass envelope making heat sinking, and cooling difficult to accomplish and control. Furthermore, commercial CCFL based LCD modules typically avoid thermally coupling CCFL lamp(s) to a system's case because the CCFL is designed to operate most efficiently when self-heated.        5. Dimming CCFL lamps, including mini CCFLs, is difficult and often expensive particularly when there is an array of four (4) or more lamps to control simultaneously, and particularly when the ambient temperature is very low as in situations of cold weather start-up. Consequently, it is uncommon for CCFL backlights to exhibit a dimming range of more than about 250:1.        
Furthermore, most of CCFLs used today include elemental mercury in their construction. Mercury is a well know biohazard that is extremely toxic to humans and the environment. For that reason many governments around the world are actively legislating to exclude these kinds of dangerous materials from commercial products. Commercial electronic products that will be affected by a mercury prohibition include flat screen TVs, flat screen computer monitors and laptop and notebook computers.
Light emitting diodes (“LEDs”) offer a near-term practical alternative to CCFLs as illumination sources for LCD backlighting. Over time LED light sources have become progressively brighter and more energy efficient while their cost continues to decline. Today, LEDs are ubiquitous in a wide array of commercial products. However, LEDs provide a small, intense and usually square or rectangular light source more accurately characterized as pseudo-point light source.
The pseudo-point light source characteristic of LEDs makes the optical problem of redistributing their light to provide an area illumination source suitable for LCD backlighting more complex than for a CCFL. Attempting to use LEDs for LCD backlighting requires that distribution optics simultaneously spread light emitted by a LED uniformly along two (2) axes compared with the requirement that light emitted by a CCFL needs to be spread essentially along only a single axis. The fact that many LED backlighting systems employ a multiplicity of sources often packaged as discrete components, possibly of different colors such as red, green and blue, exacerbates the problem of distribution optics. Consequently, any commercially practicable LED backlighting system must uniformly mix light emitted from several distinct illumination sources, and perhaps even the source's colored light, while concurrently redistributing the illumination into an area.
As described above for CCFL backlighting, the design of many modern electronic products that employ a backlit LCD require keeping the blacklight both as thin and as efficient as possible. Furthermore, modern electronic products often require a small footprint. These requirements imposed by modern electronic products further constrains the task of mixing and distributing light emitted by LEDs when used for LCD backlighting.
Lastly, many currently available backlit LCD modules employ one or two CCFL illuminators for providing light to the light pipe that in turn, more or less uniformly, redistributes the CCFL's light into an area of illumination which matches the LCD's viewable area. Accordingly, a high intensity LED blacklight illuminator for LCD modules that is capable of directly replacing CCFL illuminators and which requires no or minimal additional mechanical changes to the LCD module would be advantageous. However, since LEDs are pseudo-point light sources rather than cylindrical sources, as stated above, great care must be taken to mix into a uniform strip of illumination light emitted by LEDs that impinges upon the input surface of the LCD module's light pipe. Forming a uniform strip of illumination with light emitted from LEDs is challenging since most LCD modules provide only a few millimeter spacing between an inner edge of a frame of the LCD's module and the input surface of the module's light pipe.
Disclosure
An object of the present disclosure is to provide an improved illuminator for LCD modules.
Yet another object of the present disclosure is to provide an improved illuminator for commercial off the shelf (“COTS”) LCD modules.
Yet another object of the present disclosure is to provide an improved illuminator adapted for replacing typical CCFL illuminators included in COTS LCD modules.
Yet another object of the present disclosure is to provide an improved illuminator for LCD modules which is simply and easily manufactured.
Yet another object of the present disclosure is to provide an improved illuminator for LCD modules which facilitates maintenance and repair thereof.
LEDs offer a near-term practical alternative to CCFLs as an illumination source for LCD backlighting. The present disclosure addresses shortcomings of LEDs as an area illumination source for LCD backlighting.
Briefly, in one aspect the present disclosure includes a LCD illuminator which when energized is adapted for emitting light which impinges upon an input surface of a light pipe. As described above, the light pipe transforms light impinging thereon into an area source for backlighting a LCD of a LCD module. A LCD illuminator in accordance with the present disclosure includes a thermally conductive housing to which a thermally conductive printed circuit board (“PCB”) is mechanically and thermally bonded. The PCB has an array of LED die mounted thereon on a surface of the PCB that faces away from the thermally conductive housing. Finally, the LCD illuminator includes a layer of thermal interface material interposed between an outer surface of the thermally conductive housing and a LCD module that receives the LCD illuminator.
In another aspect the present disclosure includes an improved LCD module that has a LCD and a light pipe. The light pipe has an input surface adapted for receiving light and transforming the received light into an area source for backlighting the LCD. The improved LCD modules further includes the disclosed LCD illuminator as characterized in the immediately preceding paragraph.
An advantage of a LCD illuminator in accordance with the present disclosure is that the thin printed circuit board facilitates dissipating heat generated by LEDs included in the LCD illuminator.
Another advantage of a LCD illuminator in accordance with the present disclosure is that the thin printed circuit board spaces the LEDs further from the input surface of a light pipe which enhances mixing of light emitted by the LEDs that impinges upon the light pipe's input surface.
Yet another advantage of a LCD illuminator in accordance with the present disclosure is that it can deliver to the input aperture of an edge-lit light pipe as much as three (3) to ten (10) times the luminous flux provided by an equivalently sized CCFL illuminator.
Yet another advantage of a LCD illuminator in accordance with the present disclosure is that it permits the backlighting optics, i.e. light pipe, recycling films and rear reflector, of an LCD module to produce two (2) to three (3) times or more backlight output brightness in the same total backlight volume.
Alternatively, yet another advantage is that, if decreased system power consumption is desired rather than increased peak screen brightness, the disclosed LCD illuminator requires only one-third (⅓) to one-half (½) the power required for a CCFL illuminator that achieves the same screen brightness.
These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures.